As others have already written, there are many slightly invalid PDF files out there in the wild that many readers can display mostly fine and which your library

As others have already written, there are many slightly invalid PDF files out there in the wild that many readers can display mostly fine and which your library should also be able to handle. If you can, grab yourself a copy of the most recent PDF 2.0 specification since it contains much more information and is much more correct in terms of how to implement things. Also have a look at the errata at https://pdf-issues.pdfa.org/32000-2-2020/index.html https://pdf-issues.pdfa.org/32000-2-2020/index.html. As I'm implementing a PDF library (in Ruby), I have started to collect some situations that arise in the wild but are not spec-compliant, see https://github.com/gettalong/annotated-pdf-spec https://github.com/gettalong/annotated-pdf-spec. That might help you in parsing some invalid PDFs